PURPOSE: To contrive quickly and easily perform adjusting a clearance of a back gauge, on a supporting member of a back gauge provided on a lower die freely capable of approaching and leaving, by providing plural back gauges independently movable along with the longitudinal direction of a die respectively.

CONSTITUTION: The back gauge device 13 is composed of a stretch 15 disposed parallel to the lower die 7 and movable in front and rear directions of the machine provided with a pair of back gauges BGa, BGb capable of moving in the right and left directions (the direction Y) independently. The back gauge BGa, BGb are to control a depth of a plate material W by its ends, the plate material W controlled its depth by said back gauge BGa, BGb is bending worked into a prescribed shapes between the fixed lower die 7 and the descending upper die 11. The back gauges BGa, BGb are positioned, on the prescribed positions by motor driving respectively according to the widths, etc., of the plate material W to be bent.